Sexual Reproduction in Flowering Plants | CBSE Class 12 Biology Notes

This note covers sexual reproduction in flowering plants: the flower as the site of reproduction, the stamen and anther, microsporogenesis and the pollen grain, the pistil and the ovule, megasporogenesis and the embryo sac, the kinds and agents of pollination, outbreeding devices, pollen-pistil interaction and artificial hybridisation, double fertilisation, the endosperm and embryo, seeds and fruits, and apomixis and polyembryony.

Why is the flower the seat of sexual reproduction?

To a biologist, flowers are morphological and embryological marvels and the sites of sexual reproduction. All flowering plants (angiosperms) show sexual reproduction.

The diversity of inflorescences, flowers and floral parts shows an amazing range of adaptations. These ensure the formation of the end products of sexual reproduction, the fruits and seeds.

What happens before a flower appears?

Much before a flower is seen, the decision that the plant is going to flower has taken place. Hormonal and structural changes lead to the differentiation and development of the floral primordium. Inflorescences then bear floral buds and flowers.

In the flower, the male and female reproductive structures differentiate and develop. The androecium, a whorl of stamens, is the male reproductive organ. The gynoecium is the female reproductive organ.

The male gametophyte (pollen grain) develops in the anther, and the female gametophyte (embryo sac) develops in the ovule inside the ovary.

How are the stamen and the anther organised?

A typical stamen has a long, slender stalk, the filament, and a terminal, generally bilobed anther. The proximal end of the filament is attached to the thalamus or the petal. The number and length of stamens vary between species.

A typical angiosperm anther is bilobed, and each lobe has two theca, so it is dithecous. Often a longitudinal groove separates the theca.

In transverse section the anther is four-sided (tetragonal), with four microsporangia at the corners, two in each lobe. These develop into pollen sacs, which run the length of the anther and are packed with pollen grains.

What are the wall layers of a microsporangium?

A microsporangium appears near circular in transverse section. It is generally surrounded by four wall layers: the epidermis, endothecium, middle layers and the tapetum.

In a young anther, a group of compactly arranged, homogenous cells, the sporogenous tissue, occupies the centre of each microsporangium.

Figure 1.2 shows a stamen with a long filament and an anther, and a cut anther labelled pollen sacs, pollen grains and line of dehiscence. Figure 1.3(a) is a T.S. of a young anther labelled connective, epidermis, endothecium, middle layers, tapetum and sporogenous tissue. Figure 1.3(b) enlarges one microsporangium with microspore mother cells inside the tapetum, and 1.3(c) is a mature dehisced anther with pollen grains.

See Figs. 1.2 and 1.3 in your NCERT textbook

How are pollen grains formed?

As the anther develops, the cells of the sporogenous tissue undergo meiosis. Each of them can give rise to a microspore tetrad, so each is a potential pollen mother cell (PMC), or microspore mother cell.

Definition: Microsporogenesis is the process of formation of microspores from a pollen mother cell through meiosis.

1. The sporogenous tissue occupies the centre of each microsporangium of a young anther.
2. Each sporogenous cell acts as a pollen mother cell and divides by meiosis.
3. The microspores formed are arranged in a cluster of four cells, the microspore tetrad.
4. As the anthers mature and dehydrate, the microspores dissociate and develop into pollen grains.
5. Several thousands of pollen grains form in each microsporangium and are released when the anther dehisces.

The full developmental sequence is: sporogenous tissue, pollen mother cell, microspore tetrad, pollen grain and male gametes. The male gametes arise when the generative cell of the pollen grain divides by mitosis.

Part (a) is an enlarged pollen grain tetrad of four cells in one rounded cluster. Part (b) shows a microspore maturing: a cell with a central nucleus, one with vacuoles and a nucleus, one with an asymmetric spindle, and a pollen grain labelled vegetative cell and generative cell.

See Fig. 1.5 in your NCERT textbook

What is the structure of a pollen grain?

Pollen grains represent the male gametophytes. They are generally spherical, about 25 to 50 micrometres in diameter, with a prominent two-layered wall.

The two wall layers

The hard outer exine is made of sporopollenin, one of the most resistant organic materials known. It withstands high temperatures and strong acids and alkali, and no enzyme that degrades it is so far known.

The exine has apertures called germ pores, where sporopollenin is absent. Because of sporopollenin, pollen grains are well preserved as fossils.

The inner intine is a thin, continuous layer of cellulose and pectin. The cytoplasm is surrounded by a plasma membrane.

The cells inside

A mature pollen grain generally contains two cells. The vegetative cell is bigger, with abundant food reserve and a large, irregularly shaped nucleus. The generative cell is small, spindle shaped, with dense cytoplasm and a nucleus, and floats in the cytoplasm of the vegetative cell.

In over 60 per cent of angiosperms, pollen is shed at this 2-celled stage. In the rest, the generative cell divides mitotically into two male gametes before shedding, the 3-celled stage.

Viability, storage and allergy

Pollen must reach the stigma before it loses viability. This period is highly variable and depends partly on temperature and humidity. Rice and wheat pollen loses viability within 30 minutes, while some members of Rosaceae, Leguminoseae and Solanaceae stay viable for months.

Pollen of many species can be stored for years in liquid nitrogen (−196 °C) and used as pollen banks, like seed banks, in crop breeding.

Pollen of many species causes severe allergies and bronchial afflictions in some people, often leading to chronic respiratory disorders such as asthma and bronchitis. Parthenium (carrot grass), which came into India as a contaminant with imported wheat, is now ubiquitous and causes pollen allergy.

Pollen grains are also rich in nutrients. In western countries, many pollen products are available as tablets and syrups.

What are the parts of the pistil and the ovule?

The gynoecium may have a single pistil (monocarpellary) or more than one (multicarpellary). Multiple pistils may be fused (syncarpous) or free (apocarpous).

Each pistil has a stigma, the landing platform for pollen; a style, the slender part beneath it; and an ovary, the basal bulged part. The ovarian cavity (locule) contains the placenta.

The ovules (megasporangia) arise from the placenta. An ovary may have one ovule (wheat, paddy, mango) or many (papaya, water melon, orchids).

What is the structure of an ovule?

The ovule is attached to the placenta by a stalk, the funicle. The body of the ovule fuses with the funicle at the hilum.

One or two protective integuments encircle the nucellus except at the tip, where a small opening, the micropyle, is organised. Opposite the micropylar end is the chalaza, the basal part of the ovule.

The nucellus is a mass of cells with abundant reserve food. It contains the embryo sac, or female gametophyte. An ovule generally has a single embryo sac formed from a megaspore.

Part (a) is the pistil of Hibiscus, labelled stigma, style, ovary and thalamus. Part (b) is the syncarpous pistil of Papaver, labelled stigma and syncarpous ovary, and part (c) the apocarpous gynoecium of Michelia with many free carpels. Part (d) is an anatropous ovule labelled hilum, funicle, micropyle, micropylar pole, outer integument, inner integument, nucellus, embryo sac and chalazal pole.

See Fig. 1.7 in your NCERT textbook

How does the embryo sac develop?

Ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus. It is a large cell with dense cytoplasm and a prominent nucleus.

Definition: Megasporogenesis is the process of formation of megaspores from the megaspore mother cell.

The MMC divides by meiosis to give four megaspores. In most flowering plants, one megaspore is functional and the other three degenerate. Formation of the embryo sac from a single megaspore is called monosporic development.

1. The nucleus of the functional megaspore divides mitotically into two nuclei, which move to opposite poles (2-nucleate stage).
2. Two more sequential mitotic nuclear divisions give the 4-nucleate and then the 8-nucleate stage.
3. These divisions are strictly free nuclear: cell walls do not form immediately after them.
4. After the 8-nucleate stage, cell walls are laid down and the typical embryo sac is organised.

How are the cells arranged?

Six of the eight nuclei are organised into cells. Three at the micropylar end form the egg apparatus: two synergids and one egg cell. Three at the chalazal end are the antipodals.

The remaining two polar nuclei lie below the egg apparatus in the large central cell. So a mature embryo sac is 8-nucleate but 7-celled.

The synergids have cellular thickenings at the micropylar tip, the filiform apparatus, which guides pollen tubes into the synergid.

Part (a) shows a megaspore mother cell in the nucellus, a megaspore dyad and a megaspore tetrad. Part (b) shows the 2-, 4- and 8-nucleate stages and a mature embryo sac. Part (c) labels the antipodals at the chalazal end, the polar nuclei and central cell, and the egg, synergids and filiform apparatus at the micropylar end.

See Fig. 1.8 in your NCERT textbook

Note: The nucellus and the MMC are diploid (2n). The MMC divides by meiosis, so the functional megaspore and the embryo sac cells formed from it by mitosis are haploid (n).

What is pollination and what are its kinds?

Both male and female gametes are non-motile, so they must be brought together for fertilisation. Flowering plants use external agents for this.

Definition: Pollination is the transfer of pollen grains, shed from the anther, to the stigma of a pistil.

Autogamy

In autogamy, pollen is transferred to the stigma of the same flower. In a flower that opens and exposes its anthers and stigma, complete autogamy is rather rare. It needs synchrony of pollen release and stigma receptivity, with anthers and stigma close together.

Viola (common pansy), Oxalis and Commelina produce two types of flowers. Chasmogamous flowers have exposed anthers and stigma. Cleistogamous flowers do not open at all; their anthers dehisce in the bud, close to the stigma.

Cleistogamous flowers are invariably autogamous, as no cross-pollen can land on the stigma. They give assured seed-set even without pollinators.

Geitonogamy and xenogamy

Geitonogamy is transfer of pollen to the stigma of another flower of the same plant. It is functionally cross-pollination involving a pollinating agent, but genetically similar to autogamy.

Xenogamy is transfer of pollen to the stigma of a different plant. It is the only type that brings genetically different pollen to the stigma.

Which agents carry out pollination?

Plants use two abiotic agents (wind and water) and one biotic agent (animals). Most plants use biotic agents. In wind and water pollination, pollen reaching the stigma is a chance factor, so these flowers produce enormous amounts of pollen compared with the number of ovules.

Wind and water

Wind pollination is the more common abiotic type. Wind-pollinated flowers have light, non-sticky pollen, well-exposed stamens and a large, often-feathery stigma. They often have one ovule per ovary and many flowers packed into an inflorescence.

A familiar example is the corn cob: what is seen waving in the wind is the stigma and style, which trap pollen grains. Wind pollination is quite common in grasses.

Water pollination is quite rare, limited to about 30 genera, mostly monocotyledons. Examples are Vallisneria and Hydrilla in fresh water and marine sea-grasses such as Zostera. Most aquatic plants, such as water hyacinth and water lily, are pollinated by insects or wind.

• In Vallisneria, the female flower reaches the surface on a long stalk. Male flowers or pollen grains released on the surface drift passively, and some reach the stigma.
• In seagrasses, the female flowers stay submerged. The pollen, long and ribbon-like in many species, is carried passively inside the water.

In most water-pollinated species, a mucilaginous covering protects pollen from wetting. Wind- and water-pollinated flowers are not very colourful and do not produce nectar.

Animals

Common animal pollinators are bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) and bats. Insects, particularly bees, dominate. Some primates (lemurs), arboreal rodents and reptiles (gecko lizard and garden lizard) have also been reported as pollinators in some species.

Most insect-pollinated flowers are large, colourful, fragrant and rich in nectar; small ones are clustered into an inflorescence. Flowers pollinated by flies and beetles secrete foul odours.

Nectar and pollen are the usual floral rewards. While harvesting them, the animal comes in contact with the anthers and stigma. Its body gets coated with pollen, which is generally sticky, and when it touches a stigma it brings about pollination.

Some flowers offer safe places to lay eggs. Amorphophallus has the tallest flower, about 6 feet high. A moth and Yucca cannot complete their life cycles without each other: the moth lays eggs in the ovary locule and pollinates the flower.

Insects that consume pollen or nectar without pollinating are pollen/nectar robbers.

How do flowers discourage self-pollination?

Most flowering plants produce hermaphrodite flowers, and continued self-pollination results in inbreeding depression. Plants have therefore evolved outbreeding devices that discourage self-pollination and encourage cross-pollination.

In some species, pollen release and stigma receptivity are not synchronised. In others, the anther and stigma are placed at different positions. Both these devices prevent autogamy.

Self-incompatibility is a genetic mechanism that stops self-pollen from fertilising the ovules. It inhibits pollen germination or pollen tube growth in the pistil.

Unisexual flowers are another device. In monoecious plants such as castor and maize, male and female flowers are on the same plant. In dioecious plants such as papaya, each plant is either male or female.

What happens on the pistil after pollination?

Pollination does not guarantee the right, compatible pollen of the same species. Pollen from other species, or self-pollen in a self-incompatible plant, may also land on the stigma.

The pistil recognises the pollen. It accepts compatible pollen and promotes events leading to fertilisation, and rejects incompatible pollen by preventing germination on the stigma or tube growth in the style. This results from a chemical dialogue between pollen and pistil components.

1. The compatible pollen grain germinates on the stigma and produces a pollen tube through one of the germ pores.
2. The contents of the pollen grain move into the pollen tube.
3. The tube grows through the stigma and style to the ovary. In 2-celled pollen, the generative cell forms the two male gametes during this growth; 3-celled pollen carries them from the start.
4. The tube enters the ovule through the micropyle.
5. It enters one of the synergids through the filiform apparatus, which guides its entry.

All events from pollen deposition on the stigma until pollen tubes enter the ovule form pollen-pistil interaction. Understanding it can help breeders obtain desired hybrids, even in incompatible pollinations.

Pollen of pea, chickpea, Crotalaria, balsam or Vinca dusted on about 10 per cent sugar solution shows pollen tubes under the low power lens after about 15 to 30 minutes.

Parts (a) and (b) are photographs of pollen germinating on a stigma and pollen tubes growing through the style. Part (c) is an L.S. of a pistil showing the pollen tube reaching the ovule, labelled antipodal, polar nuclei, egg cell and synergid. Part (d) shows the tube entering a synergid, labelled filiform apparatus, male gametes and vegetative nucleus. Part (e) shows the male gametes discharged into a synergid, one moving into the egg and the other into the central cell.

See Fig. 1.12 in your NCERT textbook

How do breeders use emasculation and bagging?

Artificial hybridisation is a major approach of crop improvement. Only the desired pollen must reach the stigma, which is ensured by emasculation and bagging. If the female parent has bisexual flowers, the steps are as follows.

1. Anthers are removed from the flower bud with forceps before they dehisce (emasculation).
2. The flower is covered with a bag of suitable size, generally of butter paper, to keep out unwanted pollen (bagging).
3. When the stigma becomes receptive, mature pollen from the male parent is dusted on it.
4. The flower is rebagged and the fruits are allowed to develop.

If the female parent has unisexual flowers, emasculation is not needed. The female buds are bagged before they open, pollinated when receptive and rebagged.

What is double fertilisation?

After entering a synergid, the pollen tube releases the two male gametes into the cytoplasm of the synergid. Each takes part in a separate fusion.

1. One male gamete fuses with the nucleus of the egg cell. This is syngamy, and it forms the diploid zygote.
2. The other male gamete fuses with the two polar nuclei in the central cell.
3. This forms the triploid primary endosperm nucleus (PEN). As three haploid nuclei fuse, it is called triple fusion.
4. The central cell becomes the primary endosperm cell (PEC) and develops into the endosperm, while the zygote develops into the embryo.

As syngamy and triple fusion both occur in one embryo sac, the phenomenon is called double fertilisation, an event unique to flowering plants.

Part (a) is an embryo sac labelled degenerating synergids, zygote (2n), primary endosperm cell (PEC), primary endosperm nucleus (3n) (PEN) and degenerating antipodal cells. Part (b), drawn smaller, shows stages of a dicot embryo labelled zygote, globular embryo, heart-shaped embryo and suspensor, and a mature embryo labelled radicle, plumule and cotyledon.

See Fig. 1.13 in your NCERT textbook

How do the endosperm and the embryo develop?

Endosperm and embryo development, and the maturing of ovules into seeds and of the ovary into fruit, are the post-fertilisation events.

Endosperm

Endosperm development precedes embryo development. The PEC divides repeatedly to form triploid endosperm tissue, whose cells are filled with reserve food for the developing embryo.

Most commonly, the PEN divides repeatedly to give free nuclei, the free-nuclear endosperm. Cell walls then form and the endosperm becomes cellular. The water of a tender coconut is free-nuclear endosperm with thousands of nuclei; the white kernel is cellular endosperm.

The endosperm may be fully consumed before seed maturation (pea, groundnut, beans) or persist in the mature seed (castor, coconut) and be used during germination.

Embryo

The embryo develops at the micropylar end of the embryo sac. Most zygotes divide only after some endosperm has formed, an adaptation that gives assured nutrition to the embryo.

Early embryogeny is similar in monocots and dicots. The zygote gives rise to the proembryo, then the globular, heart-shaped and mature embryo.

Dicot and grass embryos

A dicot embryo has an embryonal axis and two cotyledons. Above the cotyledons, the epicotyl ends in the plumule (stem tip). Below them, the hypocotyl ends in the radicle (root tip), covered by a root cap.

Monocot embryos have one cotyledon, called the scutellum in grasses, lying lateral to the axis. The radicle and root cap are enclosed in the coleorrhiza, and the shoot apex and leaf primordia in the coleoptile.

Part (a) is a dicot embryo labelled plumule, cotyledons, hypocotyl, radicle and root cap. Part (b) is an L.S. of a grass embryo labelled scutellum, coleoptile, shoot apex, epiblast, radicle, root cap and coleorrhiza.

See Fig. 1.14 in your NCERT textbook

How are seeds and fruits formed?

The seed is the final product of sexual reproduction, often described as a fertilised ovule. It typically consists of seed coat(s), cotyledon(s) and an embryo axis. Cotyledons are generally thick and swollen with food reserves, as in legumes.

Non-albuminous seeds have no residual endosperm (pea, groundnut). Albuminous seeds retain some endosperm (wheat, maize, barley, castor). In black pepper and beet, a residual, persistent nucellus, the perisperm, remains.

How does the seed mature?

The integuments harden into tough, protective seed coats. The micropyle remains as a small pore that lets in oxygen and water during germination.

The seed dries to 10 to 15 per cent moisture by mass, and the embryo's metabolism slows. It may enter dormancy, or germinate if moisture, oxygen and temperature are favourable.

How does the fruit form?

The ovary develops into a fruit as the ovules become seeds. The ovary wall becomes the fruit wall, the pericarp. Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard).

In apple, strawberry and cashew, the thalamus also forms part of the fruit; these are false fruits. Fruits that develop only from the ovary are true fruits. Parthenocarpic fruits, such as banana, develop without fertilisation and are seedless; growth hormones can induce them.

Part (a) shows seeds labelled cotyledons, micropyle, seed coat, endosperm, shoot apical meristem, hypocotyl root axis and root tip, and a grain labelled pericarp, endosperm, scutellum, coleoptile, plumule, radicle and coleorrhiza. Part (b) shows apple in L.S. and T.S., labelled thalamus, seed, endocarp and mesocarp, and a strawberry labelled thalamus and achene.

See Fig. 1.15 in your NCERT textbook

Why are seeds valuable?

Seed formation is more dependable because pollination and fertilisation are independent of water. Seeds disperse to new habitats, feed seedlings until they photosynthesise, protect the embryo with a hard coat and carry new genetic combinations.

Seeds are the basis of agriculture, as dehydration and dormancy allow storage. A well-known record of a very old viable seed is that of Lupinus arcticus from the Arctic Tundra, which germinated and flowered after an estimated 10,000 years of dormancy. A 2000-year-old viable seed of the date palm, Phoenix dactylifera, was found at King Herod's palace near the Dead Sea.

What are apomixis and polyembryony?

Some flowering plants, such as some species of Asteraceae and grasses, produce seeds without fertilisation. This is apomixis, a form of asexual reproduction that mimics sexual reproduction. It differs from parthenocarpy, which produces fruits without fertilisation.

How do apomictic seeds develop?

In some species, a diploid egg cell forms without reduction division and develops into an embryo without fertilisation.

More often, as in many Citrus and mango varieties, some nucellar cells around the embryo sac divide, protrude into it and develop into embryos. Each ovule then contains many embryos.

Definition: Polyembryony is the occurrence of more than one embryo in a seed, as seen when an orange seed is squeezed.

Note: Apomictic embryos form without fertilisation, so they are genetically identical to the parent plant and can be called clones.

Why does apomixis matter for hybrid seeds?

1. Hybrid varieties have tremendously increased crop productivity.
2. Seeds collected from hybrids give progeny that segregate and lose hybrid characters, so hybrid seed must be produced every year.
3. Producing hybrid seed is costly, so the seed becomes too expensive for farmers.
4. If hybrids are made into apomicts, there is no segregation of characters, and farmers can reuse hybrid seed year after year.

Active research is going on to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.

Glossary

• Dithecous — Describes a typical angiosperm anther, which is bilobed with each lobe having two theca.
• Tapetum — The innermost wall layer of the microsporangium, whose cells have dense cytoplasm and nourish the developing pollen grains.
• Microsporogenesis — The formation of microspores from a pollen mother cell through meiosis, giving microspore tetrads.
• Sporopollenin — The material of the pollen exine, one of the most resistant organic materials known.
• Germ pore — An aperture in the pollen exine where sporopollenin is absent; the pollen tube emerges through one.
• Monosporic development — Formation of the embryo sac from a single functional megaspore, while the other three megaspores degenerate.
• Filiform apparatus — Cellular thickenings at the micropylar tip of the synergids that guide the pollen tube into a synergid.
• Cleistogamous flower — A flower that never opens, so it is invariably autogamous and gives assured seed-set.
• Geitonogamy — Transfer of pollen to the stigma of another flower of the same plant; genetically similar to autogamy.
• Self-incompatibility — A genetic mechanism that stops self-pollen fertilising the ovules by inhibiting pollen germination or tube growth.
• Triple fusion — Fusion of a male gamete with the two polar nuclei, forming the triploid primary endosperm nucleus.
• Perisperm — The residual, persistent nucellus found in some seeds, such as black pepper and beet.
• Apomixis — A form of asexual reproduction that mimics sexual reproduction, in which seeds form without fertilisation.

Common errors and misconceptions

• Misconception: A pollen grain is the male gamete. Correct: The pollen grain is the male gametophyte; its generative cell divides to form the two male gametes.
• Misconception: A mature embryo sac has eight cells. Correct: It is 8-nucleate but 7-celled, because the two polar nuclei share one central cell.
• Misconception: Geitonogamy is genetically cross-pollination. Correct: It is functionally cross-pollination but genetically similar to autogamy, as the pollen comes from the same plant.
• Misconception: Monoecy in castor and maize prevents all self-pollination. Correct: Monoecy prevents autogamy but not geitonogamy; dioecy, as in papaya, prevents both.
• Misconception: All aquatic plants are water-pollinated. Correct: In most, such as water hyacinth and water lily, flowers emerge above water and are pollinated by insects or wind.
• Misconception: The tapetum protects the anther and helps it dehisce. Correct: The outer three wall layers do that; the tapetum nourishes the developing pollen.
• Misconception: Apomixis and parthenocarpy are the same. Correct: Apomixis forms seeds without fertilisation; parthenocarpy forms seedless fruits without fertilisation.

Exam-style questions with model answers

Q1. Name the material of the pollen exine and state why pollen grains are well preserved as fossils. [2 marks]

1. Sporopollenin. It withstands high temperatures and strong acids and alkali, and no enzyme that degrades it is known, so pollen grains are well preserved as fossils.

Q2. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? [2 marks]

1. Chasmogamous flowers have exposed anthers and stigma, like flowers of other species.
2. No. Cleistogamous flowers never open, and their anthers dehisce in the bud close to the stigma, so cross-pollen cannot land on the stigma.

Q3. Why is apple called a false fruit? [2 marks]

1. In apple, the thalamus does not degenerate but contributes to fruit formation along with the ovary.
2. Fruits in which the thalamus also forms part of the fruit are false fruits; true fruits develop only from the ovary.

Q4. Arrange in developmental sequence: pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes. Name the division that forms the tetrad. [3 marks]

1. Sequence: sporogenous tissue, pollen mother cell, microspore tetrad, pollen grain, male gametes.
2. Each sporogenous cell is a potential pollen mother cell, which divides by meiosis to form a microspore tetrad (microsporogenesis).
3. As the anther dehydrates, the microspores separate and develop into pollen grains, whose generative cell divides by mitosis into two male gametes.

Q5. What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species? [3 marks]

1. Self-incompatibility is a genetic outbreeding device that prevents self-pollen, from the same flower or the same plant, from fertilising the ovules.
2. The pistil recognises the self-pollen through a chemical dialogue and rejects it.
3. It inhibits pollen germination on the stigma or pollen tube growth in the style, so the male gametes never reach the egg. Without fertilisation, no seed forms.

Q6. What is triple fusion? Where and how does it take place? [3 marks]

1. Triple fusion is the fusion of three haploid nuclei: one male gamete and the two polar nuclei.
2. It occurs in the central cell of the embryo sac.
3. The pollen tube releases two male gametes into a synergid. One fuses with the egg (syngamy); the other fuses with the polar nuclei, forming the triploid primary endosperm nucleus, which develops into the endosperm.

Q7. Describe the parts of a typical anatropous angiosperm ovule. [5 marks]

1. The ovule arises from the placenta and is attached to it by a stalk, the funicle.
2. The body of the ovule fuses with the funicle at the hilum, the junction between the two.
3. One or two integuments (outer and inner, when two are present) protect the ovule and encircle the nucellus except at the tip.
4. At the tip, a small opening, the micropyle, is organised at the micropylar pole.
5. Opposite the micropylar end is the chalaza, the basal part of the ovule.
6. The nucellus, a mass of cells with abundant reserve food, lies within the integuments.
7. The embryo sac, or female gametophyte, lies in the nucellus; there is generally one per ovule, formed from a megaspore.

Q8. Describe the formation of the female gametophyte and explain why it is 7-celled but 8-nucleate. [5 marks]

1. A single megaspore mother cell differentiates in the micropylar region of the nucellus and divides by meiosis into four megaspores.
2. In most flowering plants, one megaspore is functional and three degenerate (monosporic development).
3. Its nucleus divides mitotically into two nuclei that move to opposite poles; two more mitotic divisions give the 4-nucleate and 8-nucleate stages.
4. These divisions are free nuclear; cell walls form only after the 8-nucleate stage.
5. Three cells at the micropylar end form the egg apparatus (two synergids and an egg), and three at the chalazal end are antipodals.
6. The two remaining polar nuclei share one large central cell, so the embryo sac has 8 nuclei but 7 cells.

Q9. What is apomixis? Describe how apomictic seeds develop and explain its importance. [5 marks]

1. Apomixis is the production of seeds without fertilisation, seen in some species of Asteraceae and grasses; it is asexual reproduction that mimics sexual reproduction.
2. In some species, a diploid egg cell forms without reduction division and develops into an embryo without fertilisation.
3. More often, as in many Citrus and mango varieties, nucellar cells around the embryo sac divide, protrude into it and develop into embryos.
4. Hybrid seed must be produced every year, because the progeny of hybrid seed segregate, and this production is costly.
5. If hybrids are made into apomicts, there is no segregation, so farmers can reuse hybrid seed year after year.

Key takeaways

• A typical anther is bilobed, dithecous and tetrasporangiate; each microsporangium has an epidermis, endothecium, middle layers and a nourishing tapetum.
• Pollen mother cells divide by meiosis into microspore tetrads, which develop into pollen grains, the male gametophytes.
• The exine is made of resistant sporopollenin with germ pores; over 60 per cent of angiosperms shed pollen at the 2-celled stage.
• One functional megaspore forms the embryo sac by three free-nuclear mitotic divisions, giving a 7-celled, 8-nucleate structure.
• Only xenogamy brings genetically different pollen to the stigma; geitonogamy is functionally cross-pollination but genetically like autogamy.
• Outbreeding devices include non-synchronised maturity, separated anther and stigma, self-incompatibility, monoecy and dioecy.
• Double fertilisation, unique to flowering plants, combines syngamy forming the diploid zygote with triple fusion forming the triploid PEN.
• Apomixis forms seeds without fertilisation and could let farmers reuse hybrid seed without segregation of characters.

Test yourself

How many microsporangia does a typical anther have?

Four, located at the corners of the four-sided anther, two in each lobe.

How long do rice and wheat pollen grains stay viable?

They lose viability within 30 minutes of release, whereas some members of Rosaceae, Leguminoseae and Solanaceae stay viable for months.

What is the role of the filiform apparatus?

These thickenings at the micropylar tip of the synergids guide the pollen tube into a synergid.

Why do wind- and water-pollinated flowers make so much pollen?

Pollen reaching the stigma is a chance factor in both, so large amounts compensate for the uncertainty and the loss of pollen.

Why does the zygote stay dormant for some time?

Most zygotes divide only after some endosperm has formed, which gives assured nutrition to the developing embryo.

What do the coleoptile and coleorrhiza enclose?

The coleoptile encloses the shoot apex and leaf primordia; the coleorrhiza encloses the radicle and root cap.

What is perisperm, and where is it found?

Perisperm is the residual, persistent nucellus in some seeds, such as black pepper and beet.